A lens is a piece of a transparent substance,bounded by two surfaces, out of which one must be curved. In a lens both the surfaces may also be curved. All the distances are measured from the optic centre.The direction of the incident ray is taken as positive and the direction opposite to the incident ray is taken as negative.The refraction formula for spherical curved surface is
n2/v -n1/u = n2-n1/r and
The Gaussian formula for a lens is 1/v -1/u = 1/f
Images of different sizes and nature are formed by these surfaces of the lens and they depend on the position of the object from the refracting surfaces.
To obtain a real image of twice the size as that of the object by a converging lens of focal length 20 cm, an object must be placed at a distance of
(a) u = 20 cm
(b) u = -50 cm
(c) u =-30 cm
(d) none of these
